There's the famous phase-of-the-moon bug which struck (I believe)
Gerry Sussman and Guy Steele, then both of MIT. It turned out to
be due to code which wrote a comment into a file of LISP forms,
that included the phase of the moon as part of the text. At certain
times of the month, it would fail, due to the comment line being
longer than the "page width"; they had failed to turn off automaticnewlines that were being generated by Maclisp when the page width
was exceeded. Thus the last part of the line would be broken onto
a new line, not proceeded with a ";" (the comment character). When
reading the file back in, an error would result.

In other interpretations, one of the only ramifications of the Y2K problem on the NetHack development team.

The game was no longer reporting the phases of the moon properly (that is, in accordance with what was really going on outside at that time) and all of the subtle and enigmatic effects they had on the game were being, well, still apparent, but at the inappropriate moments that would have been correct for the year 1900.

This bug has since been fixed. It is that attention to detail which makes NetHack truly a game of champions.